1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. For example, the present invention is suitable for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, which adopts an electrophotographic process.
2. Description of the Related Art
Up to now, in an optical scanning apparatus, a light beam which is optically modulated in response to an image signal in a light source unit and emitted therefrom is periodically deflected by, for example, an optical deflector including a polygon mirror.
The light beam deflected by the optical deflector is condensed in a spot shape on a surface of a photosensitive recording medium by an imaging optical system having an fθ characteristic.
Therefore, the surface of the photosensitive recording medium is scanned with the light beam to perform image recording.
In recent years, downsizing of the entire image forming apparatus such as a laser beam printer, a digital copying machine, or a multi-function printer, and a simplification (reduction in cost) thereof are advanced.
With the advance, it is desirable to make the optical scanning apparatus more compact and simple.
Therefore, up to now, various optical scanning apparatuses designed to be made compact have been proposed (Japanese Patent Application Laid-Open No. 2001-296491 and Japanese Patent Application Laid-Open No. 2000-267030).
According to Japanese Patent Application Laid-Open No. 2001-296491, in order to shorten an interval between a deflecting surface of an optical deflector and a surface to be scanned (optical path length), a scanning field angle is increased. In addition, a shape of an imaging lens in a main scanning direction (meridional line shape), which is included in an imaging optical system, is suitably set.
Japanese Patent Application Laid-Open No. 2000-267030 discloses an optical scanning apparatus in which a converged light beam is incident on an imaging lens in a main scanning cross section.
According to Japanese Patent Application Laid-Open No. 2000-267030, in the main scanning cross section, a meridional line shape of an imaging optical system is set such that the light beam incident on the imaging optical system is refracted in a direction departing from an optical axis in a case where a scanning field angle is smaller than 77% of a maximum effective scanning field angle, and the light beam incident on the imaging optical system is refracted in a direction approaching the optical axis in a case where the scanning field angle is larger than 77% of the maximum effective scanning field angle (see FIG. 18).
In the case of Japanese Patent Application Laid-Open No. 2001-296491, in the main scanning cross section, the meridional line shape of the imaging optical system is set such that light beams incident on the imaging optical system at all scanning field angles are refracted in the direction approaching the optical axis. Therefore, when the interval is to be shortened to perform scanning with the same scanning width, it is necessary to further increase the scanning field angle.
However, when the scanning field angle is increased, an amount necessary to refract the light beams in a scanning field angle region whose scanning field angle is large becomes extremely large.
Because of this reason, there arises a problem in that an fθ characteristic, a field curvature, and other aberrations cannot be corrected in an optical system whose optical path length between the deflecting surface of the optical deflector and the surface to be scanned is short.
In the case of Japanese Patent Application Laid-Open No. 2000-267030, the meridional line shape of the imaging optical system is set such that the light beam incident on the imaging optical system is refracted in the direction departing from the optical axis in the case where the scanning field angle is smaller than 77% of the maximum effective scanning field angle.
In the imaging optical system set as described above, when the interval between the deflecting surface of the optical deflector and the surface to be scanned (optical path length) is to be further shortened, an amount necessary to refract the light beam in a direction departing from the axis in a region whose scanning field angle is small becomes extremely large. Therefore, there arises a problem in that an fθ characteristic, a field curvature, and other aberrations cannot be corrected, or the meridional line shape is distorted.
According to Japanese Patent Application Laid-Open No. 2000-267030, in the main scanning cross section, a meridional line shape of an imaging lens disposed closest to the surface to be scanned is set so as to satisfy θ2>θ3 in the entire region of the imaging lens, where θ2 denotes an angle formed between a light beam incident on the imaging lens and the optical axis and θ3 denotes an angle formed between a light beam emitted from the imaging lens and the optical axis (see FIG. 17).
An interval between a final surface of the imaging lens and the surface to be scanned (optical path length) is the longest interval among surface intervals along the optical paths between the deflecting unit and the surface to be scanned. Therefore, when the meridional line shape is set so as to satisfy θ2>θ3, an effect of shifting, from the optical axis, a position on the surface to be scanned 7 which the deflected and reflected light beam reaches, is not sufficiently obtained even at the same scanning field angle. Thus, there arises a problem in that the optical path length cannot be shortened.